Many types of electronic systems incorporate active optical devices such as laser diodes. Because they are active devices, laser diodes are prone to eventual failure. Quality control methods, such as an initial "burn-in" procedure, serve to screen out many defective laser devices. Those devices passing quality control standards are normally expected to meet a specified mean-time-to-failure operational lifetime. However, in the present art, there continue to be certain types of laser diode failures which conventional quality control screening measures are inadequate to detect.
One such type of laser diode failure has been identified and studied by the applicants. It was found that a statistically significant portion of lasers which had passed the burn-in procedure subsequently degraded in a small fraction of the expected mean-time-to-failure operational lifetime. When this problem was investigated, it appeared that the primary cause of the unexpected laser degradation was related to the presence of dark-line defects (DLDs) within the failed devices.
The phenomenon of DLDs is well-known in the relevant art. This, and various other types of laser failure modes are discussed in the referenced technical paper entitled, "Degradation of III-V Opto-Electronic Devices" by O. Ueda. In one of the failure modes, referred to as a "rapid degradation" mode, the laser device characteristically forms nonradiative regions, or DLDs, and exhibits a rapid decrease of output power. The paper attributes the formation of a DLD to a recombination-enhanced process, involving excess minority carriers, occurring at a lattice defect or dislocation present within the opto-electronic device.
At least two solutions for obtaining rapid-degradation-free devices are described in the paper by Ueda. One solution calls for chemically treating the device substrate, prior to growth of the device, so as to eliminate growth-induced lattice defects. Additional fabrication steps would be incurred with this approach. Another solution is directed to the achievement of good lattice matching and aims for the reduction of stress introduced during fabrication.
The reduction of stress in conjunction with the elimination of DLDs is also described in U.S. Pat. No. 5,173,447 issued to Ijichi et al. This reference states that a DLD may result from a condition in which the laser active layer receives a compressive stress from the substrate. In one embodiment of the invention, a stress-relieving layer is interposed between the active layer and the upper and lower clad layers. Such additional layers can be added to a laser device by a suitable epitaxial growth method, and also require additional fabrication steps.
Alternatively, Ijichi et. al. '447 teaches that lattice mismatch is reduced by adjusting the composition of upper and lower clad layers such that the relative lattice mismatch rate is less than 10.sup.-3. But this approach, which requires that the cladding layer compositions be changed, may also necessitate changes to the growth parameters of other epitaxial layers. Consequently, neither solution as taught by the reference may be a satisfactory alternative if the device fabrication procedure can not accommodate the additional steps required.
Furthermore, the reduction of lattice mismatch by the methods taught by the references is, for the most part, applicable only to laser devices based on an InGaAs/InGaAsP material system. These methods do not address the problem of DLDs arising in laser devices based on a GaAs/AlGaAs material system.
In a published technical paper entitled, "Diode Laser Degradation Mechanisms: A Review," R. G. Waters attributes laser device degradation processes to interactions occurring between excess minority carriers and lattice dislocations in the device substrate. The paper explains that these excess minority carriers are photogenerated within an operating laser device. Because lattice dislocations are present in most laser devices, such interactions between carriers and lattice can be considered a normal occurrence during lasing operation. One consequence of these interactions appears to be the generation of DLDs. A DLD often originates at a sidewall of the laser device, where a lattice dislocation resulting from a damage site is likely to reside, and propagates toward the active region, that portion of the active layer underlying an active stripe, whereupon sudden failure of the device inevitably results.
Damage sites are inherent at laser sidewalls when mechanical scribing and/or breaking is performed during the fabrication process. Laser sidewalls are commonly delineated by a scribing procedure to enable subsequent mechanical separation of adjoining devices. DLDs appear to arise from the recombination of photogenerated carriers at these damage sites. Another source of DLDs appears to be bulk defects which may be present in the substrate material used for fabrication of the laser device. But because improved growth techniques have reduced the number of such bulk defects, the proportion of DLDs resulting from bulk defects is usually smaller than the DLDs resulting from damage sites.
Two corrective methods are indicated in the Waters paper. The first method demonstrates that an etching procedure, rather than mechanical scribing, can be used to isolate or separate the individual laser devices during fabrication. A chemical etch may not produce the same type of microscopic damage sites as those produced by mechanical scribing. A chemical etching procedure is one method of avoiding mechanically-induced constituent dislocations, and the use of a chemical etch may serve to eliminate the initiation of certain types of DLDs. However, the addition of an etching operation undesirably increases the complexity of device fabrication.
The paper demonstrates that the laser sidewall edges can be displaced from the laser active region as an alternative corrective method. It has been shown that the propagation velocity of DLDs originating at a sidewall edge is reduced when the distance is increased between a laser sidewall edge and the active region. This reduction in propagation velocity is presumed to be a consequence of the smaller power density present in the semiconductor material at the increased distance from the active region.
This power density results from the effects of both the electrical current used to power the laser device and the device-generated radiation in the waveguide. It is well-known in the art that an increase in power density will increase the likelihood of DLD formation and velocity of propagation in a laser device. By displacing the laser sidewall, where damage sites are likely to be found, the damage sites are shifted to a region of lower power density and the initiation of DLDs may thereby be lessened. However, the method of sidewall displacement is not a suitable solution when constraints have been imposed on the external physical dimensions of the laser device.
Further, this method of sidewall displacement has been evaluated by the applicants who found that the method did not prevent the formation of DLDs, but that it merely increased the period of time elapsing before a DLD propagated into the laser active region and caused the device to fail. The lifetime of a defective laser was thereby extended beyond the period of time within which such a device was normally expected to fail if a DLD was present. The effectiveness of burn-in screening procedures was consequently diminished.
While the present state of the art recognizes that certain DLDs may be generated by the presence of lattice defects or stress within the laser device, the preventive measures suggested by the prior art are not suitable for all types of device configurations or fabrication methods, as explained above. Clearly, there remains a need to provide an alternative means of protection by which failures resulting from the growth of DLDs can be avoided or minimized for screened laser devices. It is therefore an object of the present invention to provide a preventative fabrication method for semiconductor devices, primarily optical radiation generating devices such as laser diodes, by which the potential occurrence of DLDs, whether attributable to constituent dislocations or stress sites in an operating device, is decreased or eliminated.
It is another object of the invention to provide such a method which allows for the mechanical separation of adjoining devices during fabrication.
It is another object of the invention to provide such a method which does not require changing the chemical constituents of the device so as to decrease or eliminate the formation of DLDs.
It is a further object of the invention to provide a laser device having a reduced probability of incurring failure due to a dark-line defect, in which the device external dimensions remain unchanged from an equivalent conventional laser device.
It is yet another object of the invention to provide such a device having the same transverse configuration of epitaxial layers as an equivalent laser device having no means for preventing failures related to dark-line defects.
Other objects of the invention will, in part, appear hereinafter and will, in part, be apparent when the following detailed description is read in connection with the drawings.